Internal combustion engines are increasingly being constructed from aluminum silicon alloys. Much research and development has been focused on the casting of such engine blocks and particularly on the silicon content of the alloys used to form such engine blocks. Lost foam and sand cast hypereutectic aluminum silicon alloy engine blocks are not used to a great extent in the automotive industry because such alloys are difficult to machine due to a presence of large primary silicon particles in the alloy microstructure. This invention describes an unexpected result with the application of pressure during the solidification of lost foam casting that decreases the primary silicon particle size and improves the machinability and wear resistance of such alloys.
In general, as aluminum cools from its liquid state, it reduces in volume. Silicon, however, expands as it cools. The laws of thermodynamics predict how the melting point of a metal changes with the application of pressure. The application of pressure to liquid aluminum, as with most metals, raises the melting point of aluminum. Again, silicon is the opposite, as the application of pressure serves to lower its melting point.
Hypereutectic aluminum silicon alloys (i.e., those alloys having greater than 11.6% silicon by weight) are conventionally used in the casting of engine blocks. Simple thermodynamic calculations indicate that the melting point of a 20% hypereutectic aluminum silicon alloy under 10 atmospheres of pressure will increase less than 1/20th of a degree. Thus, based on thermodynamic calculations, 10 atmospheres of pressure should not have an influence on the undercooling and nucleation of the silicon phase. In fact, no one has ever reported that pressure has an influence on grain size in single phase metals or in hypoeutectic aluminum silicon alloys (i.e., those alloys having less than 11.6% silicon by weight) which contain a large fraction of eutectic silicon.
Both eutectic modification and/or rapid freezing are known to increase the eutectic composition to higher silicon concentrations (e.g., from 12.6% to 15%) while simultaneously decreasing the eutectic temperature. Thus, alloys up to 15% silicon can be hypereutectic under conditions of slow cooling and no modification treatment and hypoeutectic under conditions of fast cooling or if treated with a modifier such as strontium or sodium. None of these changes are expected to be exacerbated with the application of 10 atmospheres of pressure because the melting point of the alloy is only changed by 1/20th of a degree Celsius with the application of 10 atmospheres of pressure.
With hypereutectic aluminum silicon alloys, melting point changes are expected to be even smaller than with aluminum or with silicon or with any of the hypoeutectic aluminum silicon alloys. Mondolfo, in Aluminum Alloys, reports that at 25% silicon, the volume change during a solid to liquid phase change is approximately zero. Also, since hypereutectic aluminum silicon alloys require phosphorus for the refinement of the primary silicon phase, the eutectic silicon phase is never modified and therefore the eutectic silicon phase is always coarse.
Further, each aluminum grain in the hypoeutectic aluminum silicon alloy microstructure is composed of a subset of aluminum dendrites which originate from the same nucleus. The dendrite arm spacing (DAS) is determined by the cooling rate during solidification, with faster cooling resulting in smaller values of DAS. Between the dendrite arms is the eutectic and the eutectic silicon phase is refined, if it is treated with a modifier such as strontium, or is coarse, if not treated with a modifier.
Garat, Guy & Thomas in a 1991 AFS Congress paper entitled “Solidification Under Isostatic Pressure in the Lost Foam Process” reported the DAS for both Aluminum Association Alloy Nos. 356 and 319 with and without the application of 10 atmospheres of pressure. For each group, the authors reported a DAS of 55 microns for a local solidification time of approximately 125 seconds, which would indicate that the application of pressure has no effect on the dendritic arm spacing. The authors further reported a DAS of 23 microns for a permanent mold casting having a local solidification time of 20 seconds and a DAS of 48 microns for a sand casting having a local solidification time of 100 seconds. As with grain size, there is no indication that the eutectic silicon phase or morphology is influenced by the application of pressure.
Further, the primary silicon particle size should exhibit an inverse relationship with the number of silicon nuclei present in the liquid alloy that are active during the solidification process. The time it takes to escalate the isostatic pressure enters into the process of primary silicon nucleation because it is believed that at least some nuclei require a certain time in the liquid phase before pressure assisted “good wetting” can occur and the nuclei can become active.
Thus, it appears that none of the features of hypoeutectic aluminum silicon alloy microstructure, i.e., grain size, aluminum DAS or eutectic silicon morphology, is influenced by the application of pressure either theoretically (i.e., by thermodynamic calculations) or practically (i.e., by test results in the literature).
Surprisingly, it has been recognized that the application of pressure during the solidification of a hypereutectic aluminum silicon alloy casting decreases the primary silicon particle size. Specifically, in hypereutectic aluminum silicon alloys with silicon in the range of 16% to 28% and copper in the range of 0.05% to 4.5%, a smaller primary silicon particle size has been observed when pressure is used, than when pressure is not used. The unexpected decrease in primary silicon particle size improves the machinability and wear resistance of such alloys.
The current invention improves the solidification of a hypereutectic aluminum silicon alloy casting by applying an isostatic pressure to the casting before the solidified fraction of the alloy exceeds 25% by weight. Significantly, the aluminum silicon alloy contains additives of phosphorus in the range of 0.005% to 0.1% by weight. The phosphorus reacts with the liquid aluminum in the alloy to form aluminum phosphide, a heterogeneous nucleate for primary silicon. The application of the isostatic pressure promotes more effective and extensive nucleation of the primary silicon by lowering interfacial energy and creating more favorable nucleation conditions.
With the appropriate isostatic pressure application, the method of the invention achieves a finer primary silicon particle size for hypereutectic aluminum silicon alloy compositions in the silicon range of 16 to 28% and copper in the range of 0.05 to 4.9% with magnesium in the range of 0.3 to 1.3%, and phosphorus in the range of 0.005% to 0.1%. It must be appreciated that as the silicon composition of hypereutectic aluminum silicon alloys increases, the primary silicon particles have a natural tendency to increase in size. Thus, higher amounts of phosphorus are needed and more effective nucleation is required with higher silicon concentrations in the above range. It must also be recognized that a high copper content impacts primary silicon particle size as a high copper concentration creates a larger solidification range, which in turn, creates opportunity for primary silicon floatation resulting in avoidance of nucleation and creation of large silicon structures. Further, the molten alloy may be degassed with nitrogen prior to casting. The degassing with nitrogen provides a low hydrogen content in the melt, but tends to eliminate primary silicon nuclei because all nuclei are floated out of the melt by attachment to the rising nitrogen bubbles,
The application of isostatic pressure is critically important. Higher pressures are more favorable. However, high pressures are also associated with higher manufacturing costs. Ten atmospheres has been found to be the most efficient pressure as isostatic pressures less than 5 atmospheres do not appear to be effective and isostatic pressures of 20 atmospheres appear to yield results similar to those attained with 10 atmospheres.